The present invention relates generally to silicon wafer processing, and more particularly, to Separation by Implanted OXygen” (SIMOX) silicon wafer processing.
Ion implantation techniques are particularly useful in forming a class of buried layer devices known as silicon-on-insulator (SOI) devices. In these devices, a buried insulation layer is formed beneath a thin surface silicon film. These devices have a number of potential advantages over conventional silicon devices (e.g., higher speed performance, higher temperature performance and increased radiation hardness). The lesser volume of electrically active semiconductor material in SOI devices, as compared with bulk silicon devices, tends to reduce parasitic effects such as leakage capacitance, resistance, and radiation sensitivity. In addition, the isolation between adjacent devices eliminates parasitic problems such as latch-up.
In one known technique, known by the acronym SIMOX, a thin layer of a monocrystalline silicon substrate is separated from the bulk of the substrate by implanting oxygen ions into the substrate to form a buried dielectric layer. The SIMOX process provides a heterostructure in which a buried silicon dioxide layer serves as a highly effective insulator for surface layer electronic devices.
In the SIMOX process, oxygen ions are implanted into silicon, after which the material is annealed to form the buried silicon dioxide layer or BOX region. The annealing phase redistributes the oxygen ions such that the silicon/silicon dioxide boundaries become smoother and more abrupt, thus forming a sharp and well-defined BOX region.
One important criterion for SIMOX wafers is the defect density, which should be minimized in order to produce high quality wafers. Defect density can be defined in terms of the departure from perfect crystallinity in the silicon layer that is separated from the bulk substrate by the buried oxide layer. In general, as the oxygen ions are implanted into the wafer to produce the buried SiO2 layer, atomic silicon is displaced. Additionally, excess silicon atoms from the growing BOX region can alter the crystal structure of the top silicon layer resulting in a variety of point and extended defects, such as threading dislocations and stacking faults, during the ion implantation and/or annealing processes. These defects degrade the quality and reliability of devices, e.g., transistors, that are subsequently formed in the upper silicon layer.
Hence, there exists a need for better SIMOX wafers having lower defect densities. Processes that can reduce the presence of interstitial silicon would satisfy a long felt need in the art.